1. Field of the Invention
This invention relates to an optical eaveguide used for optical communication and to a method for making the same.
2. Description of the Prior Art
Recently practical application of optical waveguides has been developed rapidly, and research and development for miniaturization and obtaining high reliability of optical parts have been pushed actively. For stabilizing a laser light source, an optical isolator made of magneto-optical material is used in a component of an optical communication system.
Prior art optical isolators are known having a fundamental construction such as that shown in FIG. 1 [e.g. Taki, Miyazaki, Akao, Denki Tsushin Gakkai Gijyutsu Hokoku (Technical Reports of the Japanese Electrical Communication Society) MW 80-95 (1981)]. That is, it is so constructed that a Bi:YIG film 2 and an upper layer made of ZnO 3 are formed by RF sputtering method on a GGG monocryptal substrate 1 and that further a mode selection circuit 4 consisting of an Al clad layer, a non-reciprocal circuit 5 and a reciprocal circuit 6 made of magnetized layers, and rutile prisms 7, 8 are added to the substrate. In this optical isolator, an incident laser beam coming through the rutile prism 7 is so influenced by Faraday effect and by Cotto-mouton effect that its polarization plane rotates and emerges from the rutile prism 8. On the other hand, since an incident laser beam coming through the rutile prism 8 is so influenced by Faraday effect and by Cotton-Mounton effect that its polarization plane further rotates, it cannot pass through the rutile prism 7.
The isolator described above is a representative example of optical isolators. However, since an optical isolator having such a construction consists of parts usually made of oxides, when it was appplied to optical intergrated circuits or integrated optics, it was not possible to combine it with a III-V compound semiconductor laser diode generally used as a light source on a III-V compound semiconductor optical waveguide in a monolithic form. this is due to the fact that the techniques for forming a monocrystal layer on an amorphous substrate on a monocrystal substrate made of a substance which is different from that of the monocrystal layer have not yet sufficiently been developed. In this respect, in order to give some examples of the prior arts methods for forming a monocrystal layer on an amorphous substrate, e.g. a method for monocrystallizing polycrystalline Si deposited in the form of an island on amorphous Si.sub.3 N.sub.4 by laser annealing method, published by J. F. Gibbons et al. [J. F. Gibbons et al., Appl. Phys. Lett., 34, 12, 831 (1979) ], and an SOI forming method by means of a splitted beam, published by Aizaki [Preliminary announcement for the 30th United Conference of Societies Related to Applied Physics in Japan, spring 1983, 6a-T-10, p. 660] can be cited.